Comment on: Diagnostic Accuracy of Optic Nerve Sheath Diameter Measured With Ocular Ultrasonography in Acute Attack of Chronic Obstructive Pulmonary Disease

Abstract

To the Editor: We really appreciate the study from Copcuoglu et al 1 about the optic nerve sheath diameter (ONSD) in patients with chronic obstructive pulmonary disease.The authors made the measurements with ocular ultrasonography, which is a fast, non-invasive tool able to assess intracranial pressure during different medical conditions.We certainly agree with this but we would like to comment some points.The authors placed a linear probe of a standard USG machine, surrounded by a sterile glove, on the closed eyelids of the patients. In our opinion this is not the best way to perform such examination. Even if this method has been used by several authors in the international literature, placing the probe over closed eyelids makes very difficult to assess the gaze direction, resulting in a wrong probe orientation and a wrong ONSD measurement. 2,3 Besides, the obtained signal strength is reduced by the glove. For these reasons, we suggest performing ocular echography with open eyelids, using methylcellulose and anesthetic drops, to visualize the patient's gaze to correctly orient the probe and without any cover that may interfere with the ultrasound.Another point that we would like to discuss is the use of a B scan probe. This technique is very useful in differentiating papilledema from Drusen that can simulate a papilledema, but it is not so effective in ONSD measurements. 4 Among the reasons there is the socalled "blooming effect" because the gain of the scan, and therefore the brightness of the image, influences ONSD measurement. With high gain the ONSD will appear smaller due to the increased wall brightness, and vice versa with low gain. This effect is particularly important as the optic nerve is such a small structure and B-scan lacks a standard sensitivity setting. 5 For the above-mentioned reasons, we would like to suggest the use of Standardized A Scan: this method displays hyperreflective spikes from the interface between arachnoid and subarachnoid fluid overcoming the blooming effect and therefore providing standardized, objective, and reliable measurements. 6